biotite, iron-rich variety of phlogopite, most abdunant of the mica minerals.

or black mica

Silicate mineral in the common mica group. It is abundant in metamorphic rocks, in pegmatites, and in granites and other igneous rocks. Biotite is a layer silicate structure in which aluminum and silicon occur in infinitely extending Si-Al-O sheets that alternate with potassium-rich and magnesium- (and iron-) containing sheets.

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